Some optical communication systems use multiple channels on parallel optical fibers for high bandwidth communications over relatively long distances, e.g., hundreds of meters to several km. A transmitter for such a system can include an optical integrated circuit having one Distributed Feedback (DFB) laser and one Electro-Absorption (EA) modulator per optical signal. In operation, the EA modulators modulate the output beams from respective DFB lasers as needed to represent transmitted data. This EA-DFB configuration, which performs modulation outside the DFB laser, can achieve data rates of 20 Gb/s and higher per optical signal. However, optical integrated circuits containing arrays of DFB laser/EA modulator pairs are complex and difficult to fabricate. As a result, the fabrication process for these systems generally has a low yield of functional integrated circuits.
These communication systems also have a general need to limit or eliminate the downstream reflections returning to the DFB lasers because such reflections can lead to instability in the optical signals output from the DFB lasers, potentially causing transmission errors. As a result, an array of optical isolators may be needed to adequately prevent down-the-line reflections from feeding back into the DFB lasers. Individual optical isolators for each laser increase system cost. Additionally, the output facets of the modulators in the integrated structure may require costly high quality anti-reflective (AR) coatings, e.g., a coating with a reflectivity less than about 10−4 to reduce reflections back into the DFB lasers.
A transmitter or other integrated circuit containing an EA-DFB array also requires drive circuits for the EA-DFB pairs of the array, and the drive currents for these devices significantly contribute to the complexity, power budget, and heating of the integrated circuit. Maintaining adequate DFB performance generally requires that the device temperature be controlled over a relatively small window. In addition, the EA modulators may also provide less than optimal performance at the extremes of the operating temperature of a high power IC. As a result, thermoelectric (TE) coolers, which are relatively inefficient devices that consume additional electrical power, may be needed.
The difficulties in fabricating and operating multiple-channel transmitters for long-distance, high-bandwidth communications make such systems expensive. Alternative high-bandwidth optical communications systems that can be produced in higher yield processes and provide high bandwidth communications over relatively long reaches and at low cost are thus sought.